In the manufacture of plastic parts, controlled cooling of the parts following formation thereof is very important to ensure that dimensional tolerances are maintained and to ensure that the parts have certain required physical properties, for example with respect to surface characteristics.
Controlled cooling of molded plastic parts can be carried out in a satisfactory manner by keeping them in the mold during the cooling period, but this is a highly inefficient use of the molding equipment. Alternatively, it has previously been found that molded or extruded plastic parts can be satisfactorily cooled by submerging them in water for a selected period of time.
A conventional apparatus which has been used for this purpose is similar to the apparatus disclosed in U.S. Pat. Nos. 4,050,575 and 4,171,044, and includes a tank which is filled with the cooling liquid and an endless belt conveyor which carries parts deposited in the tank through the liquid in the tank for a predetermined period of time and then carries the parts upwardly and out of the liquid in the tank. This apparatus has worked very well for parts which are substantially heavier than the liquid in the tank, or in other words which have a specific gravity relative to the liquid which is substantially in excess of 1.0, because these parts promptly sink in the liquid and thus promptly come to rest on the conveyor belt. This apparatus has not, however, proved satisfactory for relatively lightweight parts, namely, parts having a specific gravity which is less than or approximately equal to 1.0.
More specifically, parts which have a specific gravity less than 1.0 will float on the surface of the liquid, and parts having a specific gravity approximately equal to or slightly greater than 1.0 will, due to surface tension of the liquid, tend to float on the surface of the liquid. Accordingly, these parts are typically not immersed promptly or uniformly, and thus do not undergo cooling in the uniform manner required to assure consistent tolerances and physical properties. Moreover, since these parts do not promptly sink onto the endless conveyor belt, there is a lack of uniformity in the amount of time the parts are submerged in the liquid. In other words, the parts are not submerged in the liquid for a specific period of time in a first-in/ first-out manner, but instead spend a period of time in the liquid which is partly dependent on the time required for the part to sink, which time can hence not be precisely controlled.
Thus, other arrangements have typically been used to quench and cool relatively lightweight plastic parts. One known approach is to place the parts in a tank which is filled with a liquid and has an agitator mechanism. Agitation of the liquid causes the parts to bob around in the liquid. However, the exposure to the liquid is not as uniform as when the parts are submerged, and it has been virtually impossible with this apparatus to accurately control the amount of time each part is present in the liquid. An alternative approach has been to spray the parts with a stream of liquid, but this provides less uniform cooling of the parts than the agitator tank.
Accordingly, an object of the invention is to provide an apparatus for rapidly submersing relatively lightweight elements, including floatable elements, in a liquid in a uniform manner.
A further object of the invention is to provide an apparatus, as aforesaid, which submerses and removes parts from the liquid in a first-in/first-out manner, such that each part is immersed in the liquid for a predetermined period of time and is thus subjected to controlled cooling in a uniform manner.
A further object of the invention is to provide an apparatus, as aforesaid, which is simple, requires minimal maintenance, and is relatively inexpensive to manufacture.